It's an everyday story: boy meets girl, they fall in love and the rest is history: but not if you're a red abalone. They cast their gametes into the sea and leave it up to the sperm to do the chasing. Or do they? Richard Zimmer from the University of California, Los Angeles, explains that eggs don't leave this encounter to chance. They release an amino acid, tryptophan (Trp), to attract and guide keen sperm their way. Having previously identified tryptophan as the guidance molecule, Zimmer, Patrick Krug and Jeff Riffell focused their attention on the molecule's source, the egg, to find out more about how abalone eggs communicate with their suitors(p. 1092).
First the team decided to investigate the levels of 19 amino acids found dissolved in abalone tissue, to see if the molluscs' tryptophan levels were particularly high. But they weren't. Tryptophan levels were low in all of the abalone's tissues, except for the eggs, which had levels higher than all of the mollusc's other tissues and 3 times higher than the mollusc's ovaries. And when the team tested the eggs, they found that the amino acid was being released from the egg's cytoplasm, and not the protective surrounding gel.
Having found that the eggs are enriched with tryptophan, Zimmer was curious to find how quickly newly spawned eggs release tryptophan to attract the free-swimming sperm. Collecting freshly spawned eggs, Zimmer and Riffell placed 100 eggs into each of 13 separate Petri dishes containing fresh seawater. Collecting one seawater sample from each Petri dish at times ranging from 1 min up to 2 h, the duo handed the precious seawater samples to Patrick Krug to measure the tryptophan levels by HPLC analysis. At the same time as they took the seawater sample, Zimmer and Riffell tested the fertility of the eggs to see how it had changed as the eggs grew older. They added sperm to the eggs and incubated them for 3 h before counting how many of the eggs were fertilised successfully.
After weeks of meticulous HPLC analysis on the dilute seawater samples,Zimmer was delighted to see that the seawater's attractive tryptophan levels increased linearly up to 45 min after spawning. But then something changed. The tryptophan levels in the Petri dishes suddenly dropped. And when the team checked the eggs' fertility, they could see that it also fell dramatically at the same time. The eggs simultaneously stopped releasing tryptophan as their fertility began to decline. What was more, the tryptophan that had been released by the eggs began to vanish.
Zimmer admits that he hadn't expected this unusual tryptophan release profile and suspects that the older infertile eggs cut the tryptophan communication line to improve the chances of other fertile eggs attracting promising sperm. However, how the eggs eliminate the accumulated signal is less clear.
And having thought long and hard about why the mollusc has opted for tryptophan to attract suitors, Zimmer suggests that tryptophan could be an honest advert for the egg's fitness. Zimmer explains that tryptophan is a precursor for a key molecule involved in neural development, and suspects that releasing a strong plume of tryptophan could be the best way of advertising that the egg has a good chance of going on to produce a fine new abalone, so long as it attracts the right sperm mate.